Display devices with touch sensitivity are used today in a wide variety of applications such as touch pads in laptop computers, all-in-one computers, mobile phones and other hand-held devices, etc. It is often a desire to provide these electronic devices with a relatively large touch sensing display and still let the devices be small and thin.
There are numerous techniques for providing a display device with touch sensitivity, e.g. by adding layers of resistive wire grids or layers for capacitive touch-sensing or by integrating detectors in the display device. The major drawback of these techniques is that they reduce the optical quality of the display device, by reducing the amount of light emitted from the display or by reducing the number of active pixels of the display device.
U.S. Pat. No. 7,432,893 discloses a touch sensing system that uses FTIR (frustrated total internal reflection) to detect touching objects. Light emitted by a light source is coupled into a transparent light guide by a prism, then propagates inside the light guide by total internal reflection where after the transmitted light is received at an array of light detection points. The light may be disturbed (frustrated) by an object touching the light guide, whereby a decrease in transmitted light is sensed at certain light detection points. Providing a display device with this touch sensing system would add an undesired thickness and complexity to the display device.
WO2009/077962 also discloses a touch sensing system that uses FTIR to detect touching objects. Disclosed is a light guide with a tomograph having signal flow ports adjacent the light guide, the flow ports being arrayed around the border of the light guide. Light is emitted into the light guide by the flow ports and propagates inside the light guide by total internal reflection where after the transmitted light is detected at a plurality of flow ports. The light may be disturbed by an object touching the light guide. Providing a display device with this touch sensing system would add an undesired thickness and complexity to the display device.
US20040140960 shows a system which makes use of a different type och touch-sensing mechanism, namely by allowing beams of light to pass over the top surface of an OLED display through a prism or mirror system, and detecting obstruction of those beams. This document also proposes to use OLEDs for the light emitters. Such a design will be comparatively thick and also sensitive to contamination at the edges of the light-deflecting mechanism.
US20080150848 discloses an OLED display combined with touch sensor. In this disclosure, a separate waveguide in which infrared (IR) light propagates by TIR is placed over the display light guide, and throughout the surface of the display light guide, IR-sensing OLED elements are dispersed. Upon touching the waveguide, some light will be scattered downwards and detected by the underlying OLED sensor element. Since this solution requires IR sensors throughout the light guide, the light sensors may occupy a significant part of the display surface, hence affecting the imaging capability. The stacked solution also adds thickness to the design.